The following is not an admission that anything discussed below is citable as prior art or part of the common general knowledge.
Plant matter, for example carbohydrates or cellulose, may be fermented to produce a liquid, sometimes called beer, that is primarily water but includes ethanol. Dewatering this beer may produce ethanol that is substantially free of water, for example having less than about 1% water by volume, which may be used as a fuel or a fuel additive suitable for use in, for example, internal combustion automobile engines. Distillation can be used to partially dewater the beer, but the energy required in the distillation column reflux loop per volume percent of water removed increases as the ethanol content increases for a given number of trays in the column. At about 97% ethanol by volume, the ethanol/water azeotrope has been reached and simple distillation is no longer effective. Other techniques, such as azeotropic distillation or molecular sieves may then be used. The energy requirement of these processes is a significant problem as is the amount of water required for fermentation. Solids produced in the fermentation process, sometimes called stillage, may be useful for animal feed but must be dewatered. Dewatering involves a first step in which the stillage is dewatered typically to about a 70% moisture content measured on a dry basis and a second step in which the stillage is dried further to about a 15% moisture content measured on a dry basis as required for sale as distillers dried grains and solubles, which requires a significant amount of energy.
U.S. Pat. No. 4,978,430 describes a process in which an evaporation vessel produces a mixture of an organic compound vapour and water vapour. The mixture permeates through an aromatic polyimide gas separation membrane. The permeated vapour has an increased concentration of water vapour and a product vapour has a reduced concentration of water vapour. The permeated vapour passes through a condenser and is then returned to the evaporation vessel.
International Patent Application No. PCT/CA004/001047 filed on Jul. 16, 2004 describes an asymmetric integrally skinned membrane. The membrane can have a vapour permeance to water at least 1×10−7 mol/m2sPa at a temperature of about 30° C. to about 200° C. The membrane may have a vapour permeance selectivity of at least 50, preferably at least 250 for water/ethanol at a temperature of about 140° C. Application No. PCT/CA2004/001047 is incorporated herein in its entirety by this reference to it.